1. Field of the Invention
The present invention relates to an improved ladder hinge and locking mechanism therefor, and more particularly to a ladder hinge that has a plurality of locking positions, is structurally sound and inexpensive to manufacture.
2. The Prior Art
Ladders are commonly used for a variety of applications and are of two general types. One type is a folding laddder, commonly called a step ladder, which is self-supporting. Step ladders are typically used for such tasks as pruning, painting ceilings, or other similar tasks where it may be impossible to lean the ladder against a structure for support. The other type of ladder which is well known is the straight extension ladder. This type of ladder is simply leaned against the wall or some other structure when standing or climbing on the ladder.
Ladders which are constructed so that they may be used as both step ladders and as straight extension ladders have long been known in the art. For example, see U.S. Pat. Nos. 594,303, 1,100,823 and 3,912,043. Typically, such ladders are constructed with hinges in the middle of the side rails. The hinges permit the ladder to be folded into a step ladder configuration or unfolded into a straight extension ladder configuration. Such ladders, commonly referred to as combination step and extension ladders, are very versatile and they combine the desirable features of both types of ladders.
However, combination step and extension ladders have historically presented problems with respect to the ladder hinge. Typically, the ladder hinge is provided with two sets of support plates. Each set of support plates is rigidly attached to a respective stringer of the ladder. The support plates rotate about a central hub so that the ladder may be folded into the step ladder configuration or may be unfolded into the extension ladder configuration. Furthermore, the ladder hinge comprises a locking mechanism for locking the ladder into the various configurations. In particular, prior art devices provide a locking mechanism having spring-biased locking pins which engage apertures provided in the support plates. To accommodate the springs for biasing the locking pins, the prior art devices are provided with a central hub having a tubular cross-section. Although the ladder hinge is structurally sound as long as the locking pins engage the support plates, it has been found that the structural integrity of the ladder hinge is greatly compromised when the locking pins are withdrawn from the support plates as hereinafter set forth.
With the locking pins withdrawn from the support plate apertures, forces applied to the ladder are focused at the tubular central hub which, alone, must bear the force. Initially, the tubular hub was fashioned out of a rigid, plastic material. However, experience has shown that the rigid, plastic hub was easily broken during the folding or unfolding of the ladder from one configuration to another if any resistance to the rotation of the support plates was encountered, or if any forces were placed on the ladder hinge directly.
In an effort to overcome the breakage problem, the rigid plastic hub was replaced with a malleable, plastic hub. If forces were applied to the malleable, plastic hub, the hub was simply deformed rather than broken. However, it was found that the malleable, plastic hub eventually became permanently deformed as a result of repeated folding and unfolding. Moreover, the friction between the hub and the support plates caused grooves to be formed in the hub which gave rise to a misalignment of the locking apertures of the individual support plates. Consequently, the locking pins were able to only partially engage the ladder hinge and, thus, presented a potential danger to the user. For example, with the locking pins only partially inserted, substantial forces placed on the ladder could cause mutual shearing between the support plates and collapse of the ladder.
It would be desirable to replace the plastic support hub found in the prior art with a metal support hub thereby alleviating the problems discussed above. However, due to the configuration of the support hub (as described in some detail hereinbelow in relation to FIGS. 6A, 6B and 7), and due to the functional operation of the locking mechanism, it would be necessary to form the central hub by a die casting process if the hub were to be made of metal. When made out of plastic, the hub can be formed by injection molding or some other inexpensive plastic manufacturing process. However, it will be appreciated that substantial costs arise with the necessity of acquiring an expensive dye for metal die casting.
In an attempt to have the structural integrity of a metal hub and yet at the same time benefit from the relatively inexpensive plastic hub, a metal sleeve was placed about the exterior surface of the plastic hub at its support plate-engaging portion. However, this configuration did not prove to be a viable alternative. For example, it was found to be very difficult to rigidly secure the metal shank to the plastic hub and still allow for the proper operation of the locking mechanism. As a result, the metal shank often worked itself free from the support plates and the condition described above once again became a problem.
Another disadvantage of the prior art ladder hinge hub described above concerns the detent arrangement for holding the locking pins withdrawn until the pins are rotated relative to the hub away from the previous locked position. In the prior art device, as described below in relation to FIGS. 6A, 6B and 7, the release mechanism for the detent arrangement tends to fail due to wear and tear. The result is that the locking pins are "frozen" in the withdrawn position and cannot be inserted into the support plate apertures. This prevents locking the ladder in any discrete position, making it unsafe for use.
Accordingly, what is needed is an improved ladder hinge which may be locked into a plurality of positions, which is inexpensive to manufacture and which is structurally sound. Such an invention is disclosed and claimed herein.